Pulsed field emission cold cathode with means for replacing stripped adsorbed gas layer

ABSTRACT

The present disclosure involves methods and apparatus for continually replacing the adsorbed gas layer stripped from highcurrent field-emission cold cathodes by the effects of highvoltage pulsing the cathode.

United States Patent Samuel V. Nablo Lexington;

A. Stuart Denholm, Lincoln, both of Mass. 31,530

Apr. 24, 1970 Oct. 12, 1971 Energy Sciences, Inc.

Lexington, Mass.

[72] lnventors [21 Appl. No. [22] Filed [45] Patented [7 3] Assignee [54] PULSED FIELD EMISSION COLD CATHODE WITl-I MEANS FOR REPLACING STRIPPED ADSORBED GAS LAYER 8 Claims, 3 Drawing Figs.

[51] Int. Cl H0lj [50] FieldofSearch 313/310, 346, 209, 210, 231

[56] References Cited UNITED STATES PATENTS I 2,716,7l3 8/1955 Noel 3l3/209X Primary Examiner- David Schonberg Assistant Examiner-Toby l-l. Kusmer AttorneyRines and Rines ABSTRACT: The present disclosure involves methods and apparatus for continually replacing the adsorbed gas layer stripped from high-current field-emission cold cathodes by the effects of high-voltage pulsing the cathode.

PULSED FIELD EMISSION COLD CATHODE WITH MEANS FOR REPLACING STRIPPED ADSORBED GAS LAYER The present invention relates to electron-producing apparatus and methods, being more particularly directed to apparatus employing pulsed high-current field-emission cold cathodes.

In many applications of pulsed electron-producing apparatus, the relatively low degree of vacuum employed and/or the relatively long period between the application of successive pulses enables-the regeneration of inherent electron-ion plasma layers which serve as the electron source for high current emission. Under the action of high electric fields, these plasma layers nonnally surround high-current field-emission cold cathode surfaces employed in such apparatus. When, however, extremely high vacuum conditions obtain and/or pulses are applied at a substantial repetition rate, problems arise resulting from the exposure of the cathode material to destruction during subsequent pulsing following the removal of such inherent electron-ion plasma layers during the previous pulsing. Recent studies such applications, indeed, have shown that surface erosion rates can be sensitively controlled by the rate of replacement of the adsorbed gas layer on the conducting cathode surface. Under circumstances of high vacuum conditions, where the monolayer adsorption periods are large compared with the period between successive pulses, such erosion rates are enhanced; whereas such rates are profoundly reduced as the environmental gas pressure is increased to the point where monolayer adsorption periods become comparable with the periods between successive pulses.

In accordance with the present invention, these problems are obviated by continually supplying an appropriate fresh gas layer to the high-current field-emission cold cathode surface, such supply continuing after the purging of the conductive cathode surface by the bombardment effects of preceding pulses, including such effects as field desorption, ion sputtering negative ion formation and the like. It is thus an object of the present invention to provide a novel electron-producing apparatus and method that shall be useful, though not exclusively, in connection with the operation of electron-beam tubes at elevated pressures (e.g. 0.01 p. and greater), and in accordance with which, to limit excessive ion bombardments and electron-beam scattering associated with high pressure in general, to provide appropriate gas at the desired emitting surface or region of the cathode structure, protecting the same from destruction during continued pulsing operation.

A further object is to provide a new and improved electronemitting cathode apparatus of more general utility.

Further objects will be explained hereinafter and are more particularly delineated in the appended claims. In summary, however, from one of its aspects, the invention contemplates a novel apparatus for and method of preventing the destruction of a high-current field-emission cold cathode by pulses that remove or strip the inherent electron-ion plasma layer and the adsorbed gas from which it is formed at the highly electrically stressed cathode surface. The method comprises providing for the pulsing of the cathode, and during such pulsing, the continual supplying to the cathode surface, at desired rates, of a low atomic number, low-ionization-potential gas readily adsorbable upon the cathode surface. Preferred constructional details are hereinafter set forth.

The invention will now be described with reference to the accompanying drawings,

F IG. 1 of which is a combined longitudinal section and schematic circuit diagram illustrating the invention in preferred form; and

FIGS. 2 and 3 are longitudinal sections of modifications.

Referring to FIG. 1, an electron gun apparatus of preferred form is shown comprising an impermeable cathode shank l as, for example, of stainless steel or the like, depending from a conductive metal cap 1' and provided with a permeable terminal surface of tim 1" disposed from and spaced opposite an anode electrode 3 which may assume any conventional form.

it is here shown as a thin anodic electron-beam window 3 through which electrons may be projected as a result of the application of successive voltage pulses applied between the cathode l and anode 3, but it may also be an electron opaque target as used for X-ray production, or any other well-known form.

The pulsing circuit is schematically shown in the fonn of a capacitor bank C chargeable through an impedance R from an appropriate charging supplying source S having a negative potential terminal -V,,. This negative potential is applied, by the closing of a switch 8,, to the cathode cap 1, the opposite or left-hand terminal of capacitor(s) C being shown connected to a ground or other reference potential terminal G and to the anode structure 3. The space or acceleration gap 2 between the opposite active surfaces of the cathode 1-1" and the anode 3 is maintained in vacuum by means of any well-known dielectric-metal graded and evacuated envelope 4, schematically shown enclosing the space containing the cathode structure 1-1, the gap 2 and the upper surface of the anode 3. The envelope 4, in turn, is contained within a high-pressure insulating environment 6, such as a CO,-N, mixture or oil. It is in the high stress region 1 of the cathode 1 within the'evacuated envelope 4 that the application of pulses can result in the previously described stripping of the protective ion plasma layer; and it is at these exposed regions that the present invention contemplates replenishing fresh protective or adsorbed gas layers.

In accordance with the embodiment of FIG. 1, this end is accomplished by constructing the terminal portion 1" of the cathode l in the form of a fret or sintered-powder aggregate that is porous togas, and through the pores of which.

replenishinggas is fed from a dielectric tubular inlet 5 through the cap I and a central axial aperture in the shank l of the cathode. The gas will diffuse through the cathode terminal portion 1" as indicated by the arrows, and will provide continual replenishment of the adsorbed gas layer and associated plasma replacement layers along the localized surface 1" opposite the anode 3, thus enabling continual pulsing without exposing the cathode surface at 1 to destruction or damage. An example of a suitable porous powdered aggregate for the terminal portion 1" isa tungsten matrix. If, for example, it is desired to maintain an area of the cathode region 1" of the order of one-tenth of a square centimeter continually covered and protected by a low atomic number, low-ionization-potential gas readily adsorbable upon the surface I" (such as hydrogen, for example, which has a monolayer concentration of 1.5Xl0 cm.' despite a relatively fast pulse repetition rate of the order of, say, a hundred pulses per second, more or less, a gas flow rate of only about 5X10 cubic centimeters per second (or approximately 1 atm. cc. per minute) will be required. This is a negligible pumping load. The gas may be supplied from an ambient environment through the inlet 5, and, through the expedient of controlling the cathode conductance at the known consumption at its surface for the rated duty cycle of the pulses. [f the tungsten matrix, for example, has a conductance of about SXlO" liters/sec, for current pul ses of the order of 1000 microcoulombs at several hundred kilovolts at the above-mentioned repetition rate, adequate cathode protection can be obtained. A cylindrical tube of the same conductance (e.g., a 20 p. diameter X 5 cm. long capilla ry, connected to the atmosphere) could also be used with a high porosity tip.

While the invention has above been described with reference to hydrogen, clearly other gases of low atomic number and low ionization potential that are readily adsorbable at the cathode surface may be employed, including helium, nitrogen or alkali earth metal vapors.

In the modification of FIG. 2, an impermeable cathode shank 1 is shown provided with a modified terminal region 1", this being composed of a metal clathrate (such as, for example, zirconium hydride or palladium hydride) containing the hydrogen of similar gas in bonded, releasable form such that, following pulsing, a continual replenishment of gas layer will be provided along the external surface 1" for the protective reasons before-discussed. The very high solubilityof hydrogen in these metals (e.g., 200 cc. (STP) per gram) permits their use for the continuous supply of gas in closed or sealed systems.

An-altemate approach would utilize the normal diffusion of gases through metals as given by the relation where q is the amount of gas passing through the area A per unit of time, c is the concentration of gas at any point inside the metal and D is the diffusion constant for the metal. For a metal used as the diffusion membrane at the cathode tip, and with constant reservoir pressure of one atmosphere, the rate of diffusion increases exponentially with metal temperature. This property can provide a self-regulating feature for surface replenishment. For example, a 1 mm. thick palladium tip of 0.1 cm. area with a reservoir pressure of 1 atmosphere will deliver 7.7 micron liters per minute at a temperature of 550 C. (i.e., 0.01 atm. cc./min.), and up to 77 micron liters per minute at a temperature of 800 C. (i.e., 0.] atm. cc./min.).

Another approach to replenishing such cathode surfaces is provided by the embodiment of FIG. 3, in accordance with which the cathode is constructed in the form of a continuous loop, wire or cylinder or the like 10, movable or rotatable within a sliding seal structure 7, such as between a relatively high-pressure gas region 8 (say, 1-10 p.) and the relatively low-pressure vacuum region 2 (say, less than 0.1 u). Sealing between the highand low-pressure region can readily be accomplished by passing the wire 10 through a tight hole 7 in a disc of compressible material (neoprene, etc.). Such seals are commonly used on high-vacuum systems, often being referred to as Wilson seals. Gas is adsorbed on the cathode 10 as it is moved through the relatively high-pressure region 8 above the structure 7, so that the cathode surface is fully covered with an adsorbed gas layer by the time it has been rotated or moved down into the high-vacuum acceleration gap region 2 facing the anode 3.

Consider the time required for monolayer adsorption at the surface of such a rotating cathode, moving from high vacuum 10') into relatively high pressure at 10 torr (l p). The rate -y at which molecules strike a 1 square centimeter area may be determined by the expression where M is the molecular weight of the gas, T is its temperature, and P is the pressure in mm. This equation assumes a sticking or condensation coefficient of unity. As an illustration, assuming nitrogen is employed, wherein, at NTP, one monolayer represents 8 l0 molecules/cmF, and assuming a temperature T of 300 K., the period corresponding to this rate may be calculated to be about 2 milliseconds. This time will vary weakly with cathode temperature; i.e., T

Pore coverage in the cathode surface will proceed at a somewhat slower rate as given by Clausings relation t=3l /8a S'y where t is the time necessary to cover the pore surface in seconds, I is the pore length, (cm.), a the pore radius (cm.), S is the crosssectional area of the molecule (cm?), and 'y is the arrival rate at pressure P in molecules/cmF/see, as before defined.

For the case considered, P=10 mm., y=4.l0" em."sec. at T=300, and for [-10 cm., a==l0 cm. and S=l4 l0 cmFfor N tE5seconds.

It is evident, thus, that a high-pressure region 8 in the 1-10 micron range is entirely adequate for the replenishment of surfaces moving at rotational speeds of one cycle/second. Higher reservoir pressures and rotational speeds may be employed to satisfy the demands of applications at higher duty cycle.

Further modifications will also occur to those skilled in this art, and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

What is claimed: 1. An electron-producing apparatus having, In combination,

an anode and a cold, high-current field emission cathode; means for applying voltage pulses between the anode and cathode sufficient to draw impulses of high emission current directly from the cold cathode, the external cathode surface facing the anode being subject to the removal of inherent electron-ion plasma layer thereupon by the application of said pulses, exposing the cathode material itself to destruction upon the continued application of said pulses; and means for effectively providing a continual plasma replacement layer upon said external cathode surface comprising means for continually supplying to said surface a low atomic number lowionization-potential gas readily adsorbable upon said surface.

2. Apparatus as claimed in claim 1 and in which the said supplying means comprises a. porous cathode portion and means for applying said gas into the cathode structure and through the pores of said portion to said external surface.

3. Apparatus as claimed in claim 2 and in which said cathode has an internal gas-receiving passage communicating with said porous portion.

4. Apparatus as claimed in claim 1 and in which said cathode comprises a clathrate portion containing the said gas in bonded but releasable form.

5. Apparatus as claimed in claim 1 and in which said flowing means comprises means for moving successive portions of the cathode through a relatively high-pressure gas-containing region to adsorb gas thereupon and thence into a relatively lowpressure operative pulsing position opposite said anode.

6. Apparatus as claimed in claim 1 and in which the said gas is selected from the group consisting of hydrogen, helium, nitrogen and alkali earth metal vapors.

7. A method of preventing the destruction of a cold, high current field emission cathode by pulses that remove the inherent electron-ion plasma layer thereupon, that comprises, pulsing said cathode, and, during said pulsing, continually supplying to the cathode surface a low atomic number, lowionization-potential gas readily adsorbable upon the cathode surface.

8. A method as claimed in claim 7 and in which said supplying is effected by releasing said gas internally from a portion of said cathode. 

1. An electron-producing apparatus having, in combination, an anode and a cold, high-current field emission cathode; means for applying voltage pulses between the anode and cathode sufficient to draw impulses of high emission current directly from the cold cathode, the external cathode surface facing the anode being subject to the removal of inherent electron-ion plasma layer thereupon by the application of said pulses, exposing the cathode material itself to destruction upon the continued application of said pulses; and means for effectively providing a continual plasma replacement layer upon said external cathode surface comprising means for continually supplying to said surface a low atomic number low-ionization-potential gas readily adsorbable upon said surface.
 2. Apparatus as claimed in claim 1 and in which the said supplying means comprises a porous cathode portion and means for applying said gas into the cathode structure and through the pores of said portion to said external surface.
 3. Apparatus as claimed in claim 2 and in which said cathode has an internal gas-receiving passage communicating with said porous portion.
 4. Apparatus as claimed in claim 1 and in which said cathode comprises a clathrate portion containing the said gas in bonded but releasable form.
 5. Apparatus as claimed in claim 1 and in which said flowing means comprises means for moving successive portions of the cathode through a relatively high-pressure gas-containing region to adsorb gas thereupon and thence into a relatively low-pressure operative pulsing position opposite said anode.
 6. Apparatus as claimed in claim 1 and in which the said gas is selected from the group consisting of hydrogen, helium, nitrogen and alkali earth metal vapors.
 7. A method of preventing the destruction of a cold, high current field emission cathode by pulses that remove the inherent electron-ion plasma layer thereupon, that comprises, pulsing said cathode, and, during said pulsing, continually supplying to the cathode surface a low atomic number, low-ionization-potential gas readily adsorbable upon the cathode surface.
 8. A method as claimed in claim 7 and in which said supplying is effected by releasinG said gas internally from a portion of said cathode. 